A perpendicular magnetic recording system with recording magnetization perpendicular to the medium surface has been contemplated as an alternative to a conventional longitudinal magnetic recording system, to attain higher recording density. A perpendicular magnetic recording medium is principally composed of a magnetic recording layer of a hard magnetic material, an underlayer for aligning the magnetic recording layer to an aimed direction, a protective layer for protecting the surface of the magnetic recording layer, and an underlayer of a soft magnetic material having a function to converge a magnetic flux that is generated by a magnetic head for recording in the magnetic layer.
The soft magnetic underlayer can be omitted since recording is possible without it, although it can improve media performance. A medium without the soft magnetic underlayer is called a single-layered perpendicular magnetic recording medium, and a medium having the soft magnetic underlayer is called a double-layered perpendicular magnetic recording medium. A perpendicular magnetic recording medium, as well as a longitudinal magnetic recording medium, must perform high thermal stability compatible with low media noise in order to achieve high recording density.
In conventional longitudinal magnetic recording media, various compositions and structures of a magnetic recording layer and materials for a nonmagnetic underlayer have been proposed. Practical magnetic recording layers use an alloy of Co and Cr (hereinafter referred to as CoCr) and obtain magnetically isolated magnetic grains by segregating the chromium at the grain boundary. Another type of a magnetic recording layer, called a granular magnetic recording layer that uses nonmagnetic and nonmetallic substance, such as oxide or nitride, has been proposed.
In a magnetic recording layer of the CoCr, the substrate must be heated to a temperature higher than 200° C. during the deposition of the layer to sufficiently segregate the chromium. On the other hand, the granular magnetic recording layer has a feature where the nonmagnetic and nonmetallic substance segregates even if the substrate heating is omitted. The magnetic recording layer of CoCr and the granular magnetic recording layer can be applied to a perpendicular magnetic recording medium as well, establishing perpendicular anisotropy by controlling crystal alignment in the recording layer with the aid of an underlayer, for example.
In a perpendicular magnetic recording medium, however, it is equally difficult to segrate chromium in the magnetic recording layer using CoCr as in a longitudinal magnetic recording medium. On the hand, a perpendicular magnetic recording layer employing a granular magnetic layer makes chromium separation easier than in the CoCr recording layer. As a result, magnetic interaction between the grains can be suppressed, leading to low media noise. However, the granular magnetic recording layer in a thin film thickness of about 10 nm or less does not give sufficient segregation structure, resulting in poor isolation between grains, and causing media noise.
Because recording in a perpendicular magnetic recording medium is ideally done with a sharp perpendicular magnetic field induced by a magnetic recording head, it is desirable to form the magnetic recording layer as thin as possible. If an initial growth layer with such insufficient segregation is formed, it is difficult to obtain a useful thin magnetic recording layer. Consequently, lower noise and higher recording density have not been attained with granular magnetic recording layers.
Accordingly, there is a need to develop a perpendicular magnetic recording medium that exhibits low noise and high recording density by achieving excellent segregation structure in the magnetic layer. The present invention addresses this need.